This tradition of alluding to some well-known works of art and to the experience of the humanity on the whole roots far back in history. The ways in which reference is made differ in form, from a mere quotation up to more sophisticated ones, the example of which is allusion.
The term allusion denotes a special variety of metaphor. As the very meaning of the word shows, allusion is a brief reference to some commonly known literary or historical event. The speaker (writer) need not be explicit about what it means: he merely mentions some detail of what he thinks analogous in fiction or history to the topic discussed. Quotation is not to be mistaken for allusion, as the latter is only a hint at something, presumably known to the reader.
The poem used by George Orwell is a nursery rhyme, on which many generations of English-speaking children have been brought up. Its popularity is shown by the fact that it is listed in the book "Mother Goose's Rhymes". The names of London's major churches are rhymed in the verse. Nobody knows the origin of this poem but it first appeared in print in 1774. In its unabridged form it runs as follows:
Say The bells of old Bailey.
When I get rich.
Say The bells of Slioreditch.
When will that be?
Say The bells of Stepney.
I don't know
Say The bells of Bow.
Here comes a candle to lighT you To bed.
Here comes a chopper To chop off your head.